This invention relates to a technique for reducing the deleterious effect of titanium impurities (commonly found in metallurgical grade silicon) on silicon solar cells. It resulted from a study to determine the effects of impurities present in metallurgical grade silicon of different concentration levels, on silicon material properties and solar cell performance in order to establish the definition of solar grade silicon.
Titanium is one of the impurities commonly found in metallurgical grade silicon with concentrations as high as 10.sup.15 atoms/cm.sup.3. From previous investigations, it has been found that deliberately copper-doped N/P silicon solar cells produced from p-type material, where N is the phosphorous region and P is the boron region of the junction, and N/P copper/titanium-doped silicon solar cells from the same p-type material, have good electrical characteristics. However, the titanium-doped solar cells degraded the cell conversion efficiency to .about.61% of the undoped (baseline) silicon solar cells. To understand this behavior, a series of microstructural evaluation tests were performed on silicon wafers and cells cut from the copper-doped, titanium-doped and copper/titanium-doped crystals of concentration level 10.sup.16, 10.sup.14 and 10.sup.15 /10.sup.15 atoms/cm.sup.3 respectively.
The microstructural tests performed in this work, together with the dark I-V measurements have lead to defining the mechanism by which copper and titanium impurities and other induced crystallographic defects improve or degrade the solar cell performance.
The microstructural defects are correlated with the concentrations of the impurities, the electrical properties, and the solar cell characteristics. These correlations are necessary for establishing the concentrations that can be tolerated in the silicon single crystals without degrading the solar cell performance.
Since a diamond lattice, such as silicon, is very open, atomic diffusion is considerably easier than in close-packed lattice structures. If there is a distribution of a particular impurity between the interstitial and substitutional sites, the effective diffusion coefficient is given by a weighted combination of the two individual diffusion coefficients D.sub.i and D.sub.s : EQU D.sub.eff =D.sub.i f+D.sub.s (1-f)
where f is a fraction in interstitial sites.
It has been proved that copper behaves in this manner by R. N. Hall and H. J. Racette, "Diffusion and Solubility of Copper in Extrinsic and Intrinsic Ge, Si and GaAs," J. Appl. Phys, vol. 35(2), 379, 1964, making precipitation or gettering an easy matter. Copper is reported to diffuse interstitially as an electrically active single charged positive ion (donor). Interstitial copper ions combine with vacancies in the silicon lattice to create a substitutional impurity. Substitutional copper has been identified as a triple acceptor in silicon crystals. (See Hall and Racette, supra.) Substitutional impurities are relatively immobile, and consequently, the effective diffusion rate of the impurity is determined by the relative abundance of the interstitial or substitutional species. Furthermore, the migration of impurities depend strongly upon the imperfection of the host silicon crystal, i.e. presence of vacancies.
Using electron spin Resonance (ESR) measurements, titanium was found to have diffused interstitially in silicon. Also, titanium is known to have a high affinity to react with oxygen and form a wide range of titanium oxides. (See Metals Handbook volume I on "Titanium and Titanium Alloys," pp. 1147-1153, 8th Edition, American Society for Metal, 1961.) Therefore, there is a great tendency for the titanium to react with the oxygen in the Czochralski grown crystal (during growth or annealing) and form interstitial oxides.
For a device such as the solar cell, the quality of the p-n junction is intimately related to the structural perfection of the junction depletion region. Crystal growth and device processing steps, both introduce in the silicon, structural imperfections such as precipitates and dislocations during the diffusion process (strain effects caused by the diffusing species) and oxide precipitate associated with the strain effects. The structural imperfections provide traps capable of storing and immobilizing the electrical charges. Accordingly, degradation of the electrical behavior of the solar cells might occur. The type of adverse electrical behavior is determined by the type, the size, and the distribution of defects. According to W. Shockley, "The Theory of P-n Junctions in Semiconductors and P-n Junctions," Bell Syst. Tech. J, Vol. 28, p. 435, 1949, metal precipitates in the space charge region are expected to produce soft junction characteristics since they are conducting.
Basically, those defects affect the electrical behavior of a diode in the following ways:
(a) Decrease the reverse voltage. PA1 (b) Increase the reverse current. PA1 (c) Cause forward or reverse tunneling. PA1 (d) Decrease the minority carrier lifetime. PA1 (e) In the case of solar cells, decrease the power output and fill factor and degrade the cell efficiency.
The silicon wafers and cells subjected to evaluation tests were 0.025 cm thick, and 1.times.2 cm.sup.2 in size. The wafers were cut from ingots of diameter 3.12 cm prepared by Czochralski techniques with &lt;111&gt; growth axis. The impurities (Cu, Ti) were added to the ingots during the melting process at 1412.degree. C. Some of these wafers were characterized as received and then characterized after phosphorous diffusion. Solar cells (1.times.2 cm.sup.2 cells) fabricated from this material were evaluated electrically and microscopically using charge collection microscopy.
In order to investigate the effect of the defects associated with the deliberately added impurities (Cu, Ti and Cu/Ti), nondestructive experiments were performed on the as-grown wafers and after the phosphorous diffusion. Another set of experiments was performed on the finished solar cells to determine the perfection of the junction.
By comparing the results of the two sets of experiments (before and after processing), it is possible to track the critical step for the defect generation and define the defect that causes the junction degradation.